The invention relates to a dual wavelength semiconductor laser source for an optical pickup having a multiplexing waveguide formed in a photonic crystal.
Since a recording density of information (data) recorded on a digital video disk (DVD) is comparatively high, an AlGaInP-based semiconductor laser element having an emission wavelength in a 600 nm band, for example, of 650 nm is used as a laser source to reproduce the information.
An optical pickup used in a prior art DVD device, however, was unable to reproduce data recorded on a compact disk (CD) and a minidisk (MD) using an AlGaAs-based semiconductor laser element having an emission wavelength in a 700 nm band, for example, of 780 nm.
Therefore, as an optical pickup light source capable of reproducing both of DVD and CD or MD, there has been used a dual wavelength semiconductor laser source including an AlGaInP-based semiconductor laser element having an emission wavelength in a 600 nm band and an AlGaAs-based semiconductor laser element having an emission wavelength in a 700 nm band.
In this case, an AlGaInP-based semiconductor laser element and an AlGaAs-based semiconductor laser element are incorporated in one package and an optical pickup integrated into one piece as a dual wavelength semiconductor laser source inevitably becomes larger in size. As a result, since a DVD device itself becomes larger in size, a problem arises that down-sizing is disabled.
Therefore, in order to make an optical pickup smaller in size, an optical pickup having a dual wavelength semiconductor laser element has been known, as disclosed in Laid Open Japanese Patent Application Publication No. 11-186651, in which, for example, an AlGaInP-based semiconductor laser element having an emission wavelength in a 600 nm band and an AlGaAs-based semiconductor laser element having an emission wavelength in a 700 nm band are separately formed and monolithically integrated on a single semiconductor substrate made of GaAs to obtain the dual wavelength semiconductor laser element.
As disclosed in the above publication, even with a down-sized integrated dual wavelength semiconductor laser element adopted, a necessity arises for additional usage of other optical parts together with the semiconductor laser elements in order to realize a small size and low cost in an optical pickup. As a result, even in a down-sized integrated dual wavelength semiconductor laser element, a problem occurs that complexity is encountered in an optical system design and reduction in utilization of output light is entailed since light emitting points of two lasers are spaced apart from each other.
In order to realize a dual wavelength semiconductor laser source with a single light emitting point, though an optical waveguide formed in a compound semiconductor substrate is used to multiplex beams having two wavelengths, for example with a Y-shaped waveguide to thereby enable a single light emitting point to be formed, a multiplexing waveguide becomes much larger in size as compared with semiconductor laser elements when a light loss caused by the multiplexing waveguide is intended to be smaller with common type optical waveguides.
For example, reduction in optical loss using a Y-shaped multiplexing waveguide would require an intersecting angle of two input waveguides of 3 degrees or less and in addition, the total length of the Y-shaped multiplexing waveguide would amount to 2 mm or more with a spacing between two light emitting points of 100 μm of an integrated dual wavelength semiconductor laser element. This value is problematic in being not suitable for down-sizing of an optical pickup since it is very large as compared with a size of an integrated dual wavelength semiconductor laser element only having a length and width of the order of 300 μm each.